Restraint of enzymolysis and photolysis of organic phosphorus and pyrophosphate using synthetic zeolite with humic acid and lanthanum

Although phosphorus (P)-removing materials have been widely used to immobilize orthophosphate in aquatic ecosystems, the immobilization of organic phosphorus (OP) and pyrophosphate (PP) remains unclear. In this study, a unique OP/PP-immobilizing synthetic zeolite loaded with humic acid (HA) and La(OH)3 nanorod composite (ZHLA) is synthesized, and OP/PP-immobilization is examined by means of enzymolysis and photolysis. The results indicate that OP and PP could not be photodegraded into PO43− under natural conditions. Although OP could be degraded into PO43− by enhanced Fenton’s reagent, La3+ and HA could protect OP and PP from degradation. As a result of the colloidal coagulation and photocatalysis of La(OH)3, the activity of phosphatase could be reduced. The PP and phytate become resistant to enzymolysis through the formation of stable HA–La–PP/phytate or La–PP/phytate chelates. However, labile monoester phosphate could not form stable chelates resisting to enzymolysis. This labile monoester phosphate is hydrolyzed into PO43− and thereafter immobilized by ZHLA, which is the main pathway to achieve permanent immobilization. Most release-sensitive P fractions are transferred to the stable P fraction after ZHLA application. The ZHLA is demonstrated to be an economical and effective P-immobilization material and has high potential as a commercial sediment-capping product for eutrophication control.